Older birds most likely to get strokes, however I had a 13 month old AG that died of a stroke. Gigi showed none of the signs below, just fell over and died.

Signs include paralysis, the bird not being able to sit on a perch or swing, not being able to fly and screaming. One side of the bird is affected, even being blind in just one eye. The bird will become disoriented, panic and be hard to calm down. The bird may also have a loss of appetite. If the bird does not respond to medication within a week, it is not likely to at all.

My moms 40+ y/o green cheek amazon had a stroke last summer. He just came crashing off his perch in the dead of night and was totally limp on his left side. It was kind of a shock when the vet told her he had suffered a stroke, it's not really something one would think would happen to a parrot. He has been recovering quite well, and is able to perch and take short flights again (he spent a few months with no perches because he would fall and wasn't allowed to fly). I just saw him yesterday, and he looks pretty good, his left wing is still a little droopy and his foot a little curled, but he's getting around fine and pretty much back to himself.

Strokes in parrots probably happen more often than they are recorded, as many birds would probably just fall over dead and no one would ever know the reason was a stroke. Just like humans, I would assume strokes are more likely in older birds than younger ones, or birds with unhealthy diets. A proper diet and reasonable amounts of activities are the best preventative measures of any disease or medical problems, but sometimes things just happen (Barney has a healthy diet and is very active, but he still had a stroke). I don't think it's something you really need to worry about if your feeding your bird right and they are an active parrot. If he/she does have a stroke, there wouldn't be anything else you could've done to prevent it, and it's probably still a relatively rare thing anyways.